Finish https://codereview.chromium.org/1948763003

device/bluetooth/bluetooth_low_energy_device_mac.mm

Index: device/bluetooth/bluetooth_low_energy_device_mac.mm
diff --git a/device/bluetooth/bluetooth_low_energy_device_mac.mm b/device/bluetooth/bluetooth_low_energy_device_mac.mm
index 0573c2fc11b8cb11d9e70db97c3c4e87b78518d8..20d2a4cfd480aafb469390ef6107b5fda013bd13 100644
--- a/device/bluetooth/bluetooth_low_energy_device_mac.mm
+++ b/device/bluetooth/bluetooth_low_energy_device_mac.mm
@@ -252,8 +252,9 @@ void BluetoothLowEnergyDeviceMac::DidDiscoverPrimaryServices(NSError* error) {
     if (!gatt_service) {
       gatt_service = new BluetoothRemoteGattServiceMac(this, cb_service,
                                                        true /* is_primary */);
-      gatt_services_.add(gatt_service->GetIdentifier(),
-                         base::WrapUnique(gatt_service));
+      auto result_iter = gatt_services_.add(gatt_service->GetIdentifier(),
+                                            base::WrapUnique(gatt_service));
+      DCHECK(result_iter.second);
       adapter_->NotifyGattServiceAdded(gatt_service);
     }
   }